The genetic basis for most patients with pustular skin disease remains elusive.

BACKGROUND: Rare variants in the genes IL36RN, CARD14 and AP1S3 have been identified to cause or contribute to pustular skin diseases, primarily generalized pustular psoriasis (GPP). OBJECTIVES: To better understand the disease relevance of these genes, we screened our cohorts of patients with pustular skin diseases [primarily GPP and palmoplantar pustular psoriasis (PPP)] for coding changes in these three genes. Carriers of single heterozygous IL36RN mutations were screened for a second mutation in IL36RN. METHODS: Coding exons of IL36RN, CARD14 and AP1S3 were sequenced in 67 patients - 61 with GPP, two with acute generalized exanthematous pustulosis and four with acrodermatitis continua of Hallopeau. We screened IL36RN and AP1S3 for intragenic copy-number variants and 258 patients with PPP for coding changes in AP1S3. Eleven heterozygous IL36RN mutations carriers were analysed for a second noncoding IL36RN mutation. Genotype-phenotype correlations in carriers/noncarriers of IL36RN mutations were assessed within the GPP cohort. RESULTS: The majority of patients (GPP, 64%) did not carry rare variants in any of the three genes. Biallelic and monoallelic IL36RN mutations were identified in 15 and five patients with GPP, respectively. Noncoding rare IL36RN variants were not identified in heterozygous carriers. The only significant genotype-phenotype correlation observed for IL36RN mutation carriers was early age at disease onset. Additional rare CARD14 or AP1S3 variants were identified in 15% of IL36RN mutation carriers. CONCLUSIONS: The identification of IL36RN mutation carriers harbouring additional rare variants in CARD14 or AP1S3 indicates a more complex mode of inheritance of pustular psoriasis. Our results suggest that, in heterozygous IL36RN mutation carriers, there are additional disease-causing genetic factors outside IL36RN.

Evidence for genetic overlap between adult onset Still's disease and hereditary periodic fever syndromes.

OBJECTIVE: Adult onset Still's disease (AOSD) is a severe, autoimmune disease that can be challenging to treat with conventional therapeutics and biologicals in a considerable number of cases. Therefore, there is a high need to understand its pathogenesis better. As major clinical symptoms overlap between AOSD and hereditary periodic fever syndromes (HPFS), we analysed four known HPFS genes in AOSD. METHODS: We performed Sanger sequencing and quantitative analysis of all coding regions of MEFV, TNFRSF1A, MVK and NLRP3 in 40 AOSD patients. All rare coding variants (n = 6) were evaluated for several aspects to classify them as benign to pathogenic variants. Statistical analysis was performed to analyse whether variants classified as (likely) pathogenic were associated with AOSD. RESULTS: We identified three rare variants in MEFV, one previously not described. Association to the three likely pathogenic MEFV variants was significant (p c = 2.34E- 03), and two of the three carriers had a severe course of disease. We observed strong evidence for significant association to mutations in TNFRSF1A (p c = 2.40E- 04), as 5% of patients (2/40) carried a (likely) pathogenic variant in this gene. Both of them received a biological for treatment. CONCLUSION: Our results indicate TNFRSF1A as a relevant gene in AOSD, especially in patients with a more challenging course of disease, while causal variants remain to be identified in the majority of patients.

Phenotypic variability in a large Czech family with a dynamin 2-associated Charcot-Marie-Tooth neuropathy.

Mutations in the Dynamin 2 gene (DNM2) cause autosomal dominant centronuclear myopathy or autosomal dominant (AD) Charcot-Marie-Tooth (CMT) disease. Here the authors report one large Czech family with 15 members affected with an AD CMT phenotype of extraordinary variability. Genetic linkage analysis using SNP arrays revealed a locus of about 9.6 Mb on chromosome 19p13.1-13.2. In this critical interval, 373 genes were located. The only gene herein known to be associated with an intermediate type of CMT was Dynamin 2 (DNM2). Subsequent sequence analysis of the DNM2 gene in the index patient revealed a novel missense mutation p.Met580Thr. This missense mutation segregated with the neuropathy, indicating the causal character of this mutation. The phenotype of CMT in this family shows mild to moderate impairment with relatively preserved upper limbs and a very broad range of the onset of clinical symptoms from an early onset around the age of 12 to the late onset during the fifth decade. Electrophysiology showed an intermediate type of peripheral neuropathy. The motor median nerve conduction velocity varied from 36 m/s to normal values with signs of asymmetrical affection of peripheral nerves. No additional symptoms such as cranial nerve involvement, cataract, and signs of neutropenia or myopathy syndrome were observed in any member of the family yet. The progression was slow with no loss of ambulation. The authors suggest that the characterization of clinical variability in a single family may help to direct the genetic analysis directly to the rarely observed DNM2 mutations.

Structure of the equine infectious anemia virus Tat protein.

Trans-activator (Tat) proteins regulate the transcription of lentiviral DNA in the host cell genome. These RNA binding proteins participate in the life cycle of all known lentiviruses, such as the human immunodeficiency viruses (HIV) or the equine infectious anemia virus (EIAV). The consensus RNA binding motifs [the trans-activation responsive element (TAR)] of HIV-1 as well as EIAV Tat proteins are well characterized. The structure of the 75-amino acid EIAV Tat protein in solution was determined by two- and three-dimensional nuclear magnetic resonance methods and molecular dynamics calculations. The protein structure exhibits a well-defined hydrophobic core of 15 amino acids that serves as a scaffold for two flexible domains corresponding to the NH2- and COOH-terminal regions. The core region is a strictly conserved sequence region among the known Tat proteins. The structural data can be used to explain several of the observed features of Tat proteins.

Further delineation of Pitt-Hopkins syndrome: phenotypic and genotypic description of 16 novel patients.

BACKGROUND: Haploinsufficiency of the gene encoding for transcription factor 4 (TCF4) was recently identified as the underlying cause of Pitt-Hopkins syndrome (PTHS), an underdiagnosed mental-retardation syndrome characterised by a distinct facial gestalt, breathing anomalies and severe mental retardation. METHODS: TCF4 mutational analysis was performed in 117 patients with PTHS-like features. RESULTS: In total, 16 novel mutations were identified. All of these proven patients were severely mentally retarded and showed a distinct facial gestalt. In addition, 56% had breathing anomalies, 56% had microcephaly, 38% had seizures and 44% had MRI anomalies. CONCLUSION: This study provides further evidence of the mutational and clinical spectrum of PTHS and confirms its important role in the differential diagnosis of severe mental retardation.

The chemical class of quinazoline compounds provides a core structure for the design of anticytomegaloviral kinase inhibitors.

HCMV is a member of the family Herpesviridae and represents a worldwide distributed pathogen with seropositivity rates in the adult population ranging between 40% and 90%. Notably, HCMV infection is a serious, sometimes life-threatening medical problem for newborns and immunosuppressed individuals, including transplant recipients and patients under antitumoral chemotherapy. Current standard therapy with valganciclovir has the disadvantage of inducing drug-resistant virus mutants and toxicity-related side effects. Our analysis stresses the earlier finding that kinase inhibitors of the quinazoline class exert an antiviral response by targeting the viral protein kinase pUL97 without inducing resistance. Therefore, quinazolines have been used as a core structure to gain insight in the mode of inhibitor-kinase interaction. Here, we demonstrate that (i) the novel quinazolines Vi7392 and Vi7453 are highly active against HCMV laboratory and clinically relevant strains including maribavir- and ganciclovir-resistant variants, (ii) antiviral activity is not cell-type specific and was also detected in a placental explant tissue model using a genetically intact HCMV strain (iii) the viral kinase pUL97 represents a target of the anticytomegaloviral activity of these compounds, (iv) induction of pUL97-conferring drug resistance was not detectable under single-step selection, thus differed from the induction of ganciclovir resistance, and (v) pUL97 drug docking simulations enabled detailed insights into specific drug-target binding properties providing a promising basis for the design of optimized kinase inhibitors. These novel findings may open new prospects for the future medical use of quinazoline drug candidates and the use of drug-target dynamic simulations for rational design of antivirals.

The structure of iron-sulfur proteins.

Ferredoxins are a group of iron-sulfur proteins for which a wealth of structural and mutational data have recently become available. Previously unknown structures of ferredoxins which are adapted to halophilic, acidophilic or hyperthermophilic environments and new cysteine patterns for cluster ligation and non-cysteine cluster ligation have been described. Site-directed mutagenesis experiments have given insight into factors that influence the geometry, stability, redox potential, electronic properties and electron-transfer reactivity of iron-sulfur clusters.

Solution structure of the glycosylated second type 2 module of fibronectin.

Fibronectin is an extracellular matrix glycoprotein that plays a role in a number of physiological processes involving cell adhesion and migration. The modules of the fibronectin monomer are organized into proteolytically resistant domains that in isolation retain their affinity for various ligands. The tertiary structure of the glycosylated second type 2 module (2F2) from the gelatin-binding domain of fibronectin was determined by two-dimensional nuclear magnetic resonance spectroscopy and simulated annealing. The structure is well defined with an overall fold typical of F2 modules, showing two double-stranded antiparallel beta-sheets and a partially solvent-exposed hydrophobic cluster. An N-terminal beta-sheet, that was not present in previously determined F2 module structures, may be important for defining the relative orientation of adjacent F2 modules in fibronectin. This is the first three-dimensional structure of a glycosylated module of fibronectin, and provides insight into the possible role of the glycosylation in protein stability, protease resistance and modulation of collagen binding. Based on the structures of the isolated modules, models for the 1F22F2 pair were generated by randomly changing the orientation of the linker peptide between the modules. The models suggest that the two putative collagen binding sites in the pair form discrete binding sites, rather than combining to form a single binding site.

Equine infectious anemia virus transactivator is a homeodomain-type protein.

Lentiviral transactivator (Tat) proteins are essential for viral replication. Tat proteins of human immunodeficiency virus type 1 and bovine immunodeficiency virus form complexes with their respective RNA targets (Tat responsive element, TAR), and specific binding of the equine anemia virus (EIAV) Tat protein to a target TAR RNA is suggested by mutational analysis of the TAR RNA. Structural data on equine infectious anemia virus Tat protein reveal a helix-loop-helix-turn-helix limit structure very similar to homeobox domains that are known to bind specifically to DNA. Here we report results of gel-shift and footprinting analysis as well as fluorescence and nuclear magnetic resonance spectroscopy experiments that clearly show that EIAV Tat protein binds to DNA specifically at the long terminal repeat Pu.1 (GTTCCTGTTTT) and AP-1 (TGACGCG) sites, and thus suggest a common mechanism for the action of some of the known lentiviral Tat proteins via the AP-1 initiator site. Complex formation with DNA induces specific shifts of the proton NMR resonances originating from amino acids in the core and basic domains of the protein.

Whole exome sequencing reveals a novel de novo FOXC1 mutation in a patient with unrecognized Axenfeld-Rieger syndrome and glaucoma.

We report the identification of a novel mutation in the fork-head box C1 (FOXC1) gene which occurred de novo in an Italian patient with unrecognized Axenfeld-Rieger syndrome. He was previously diagnosed as having late recognized primary congenital glaucoma at the age of 14 years and was subsequently subjected to multiple surgical interventions due to uncontrolled intraocular pressure and progressive visual field loss. After exclusion of mutations in CYP1B1 and MYOC, trio-whole-exome sequencing revealed de novo in frame deletion in the coding region of the FOXC1 gene (c.407_409delGTC, p.V137del) leading to a deletion of the evolutionary conserved amino acid Valine at position 137 of the protein. Molecular modeling predicted that Val137 deletion impairs FOXC1 DNA-binding capacity and transcriptional activation. Since loss-of-function mutations in FOXC1 are associated with Axenfeld-Rieger syndrome, the genetic findings in combination with re-evaluation of the patient's clinical data resulted in a corrected diagnosis of Axenfeld-Rieger syndrome with developmental glaucoma. We therefore suggest that in addition to CYP1B1 and MYOC, FOXC1 should be included in the genetic analysis of cases with unclear glaucomatous phenotypes to ensure proper diagnosis, adequate treatment and appropriate genetic counseling.

Solution structure of a zinc substituted eukaryotic rubredoxin from the cryptomonad alga Guillardia theta.

The rubredoxin from the cryptomonad Guillardia theta is one of the first examples of a rubredoxin encoded in a eukaryotic organism. The structure of a soluble zinc-substituted 70-residue G. theta rubredoxin lacking the membrane anchor and the thylakoid targeting sequence was determined by multidimensional heteronuclear NMR, representing the first three-dimensional (3D) structure of a eukaryotic rubredoxin. For the structure calculation a strategy was applied in which information about hydrogen bonds was directly inferred from a long-range HNCO experiment, and the dynamics of the protein was deduced from heteronuclear nuclear Overhauser effect data and exchange rates of the amide protons. The structure is well defined, exhibiting average root-mean-square deviations of 0.21 A for the backbone heavy atoms and 0.67 A for all heavy atoms of residues 7-56, and an increased flexibility toward the termini. The structure of this core fold is almost identical to that of prokaryotic rubredoxins. There are, however, significant differences with respect to the charge distribution at the protein surface, suggesting that G. theta rubredoxin exerts a different physiological function compared to the structurally characterized prokaryotic rubredoxins. The amino-terminal residues containing the putative signal peptidase recognition/cleavage site show an increased flexibility compared to the core fold, but still adopt a defined 3D orientation, which is mainly stabilized by nonlocal interactions to residues of the carboxy-terminal region. This orientation might reflect the structural elements and charge pattern necessary for correct signal peptidase recognition of the G. theta rubredoxin precursor.

Structure determination of human and murine beta-defensins reveals structural conservation in the absence of significant sequence similarity.

Defensins are cationic and cysteine-rich peptides that play a crucial role in the host defense against microorganisms of many organisms by their capability to permeabilize bacterial membranes. The low sequence similarity among the members of the large mammalian beta-defensin family suggests that their antimicrobial activity is largely independent of their primary structure. To investigate to what extent these defensins share a similar fold, the structures of the two human beta-defensins, hBD-1 and hBD-2, as well as those of two novel murine defensins, termed mBD-7 and mBD-8, were determined by nuclear magnetic resonance spectroscopy. All four defensins investigated share a striking similarity on the level of secondary and tertiary structure including the lack of a distinct hydrophobic core, suggesting that the fold is mainly stabilized by the presence of three disulfide bonds. In addition to the overall shape of the molecules, the ratio of solvent-exposed polar and hydrophobic side chains is also very similar among the four defensins investigated. It is significant that beta-defensins do not exhibit a common pattern of charged and hydrophobic residues on the protein surface and that the beta-defensin-specific fold appears to accommodate a wide range of different amino acids at most sequence positions. In addition to the implications for the mode of biological defensin actions, these findings are of particular interest because beta-defensins have been suggested as lead compounds for the development of novel peptide antibiotics for the therapy of infectious diseases.

Identification and characterization of a eukaryotically encoded rubredoxin in a cryptomonad alga.

We have identified an open reading frame with homology to prokaryotic rubredoxins (rds) on a nucleomorph chromosome of the cryptomonad alga Guillardia theta. cDNA analysis let us propose that the rd preprotein has an NH(2)-terminal extension that functions as a transit peptide for import into the plastid. Compared to rds found in non-photosynthetic prokaryotes or found in bacteria that exhibit an anoxigenic photosynthesis apparatus, nucleomorph rd has a COOH-terminal extension, which shows high homology exclusively to the COOH-termini of cyanobacterial rds as well as to a hypothetical rd in the Arabidopsis genome. This extension can be divided into a putative membrane anchor and a stretch of about 20 amino acids with unknown function linking the common rd fold to this anchor. Overexpression of nucleomorph rd in Escherichia coli using a T7 RNA polymerase/promotor system resulted in a mixture of iron-containing holorubredoxin and zinc-substituted protein. Preliminary spectroscopic studies of the iron form of nucleomorph rd suggest the existence of a native rd-type iron site. One-dimensional nuclear magnetic resonance spectroscopy of recombinant Zn-rd suggests the presence of a stable tertiary fold similar to that of other rd structures determined previously.

Molecular dynamics simulation of equine infectious anemia virus Tat protein in water and in 40% trifluoroethanol.

Two molecular dynamics (MD) simulations were performed in order to increase the understanding of the dependence of protein conformation on solvent environment. The protein used for these simulations is the transcriptional activator of the equine infectious anemia virus (EIAV-Tat). The structure of this protein has been determined by nuclear magnetic resonance (NMR) in aqueous solution (Willbold et al., Science 264, 1584 (1994)) and in 40% (v/v) trifluoroethanol (TFE) (Sticht et al., Eur. J. Biochem., submitted) showing considerable differences in the stability of the secondary structure elements. In order to investigate the influence of the solvent MD simulations (300 K: 200 ps) were carried out in water and in a solvent containing 40% (v/v) TFE. In both simulations the structure as determined in 40% TFE by NMR showing three-helices and a tight type II turn, was used as the initial structure. The MD simulations clearly indicate a decreased stability of the secondary structure elements in aqueous environment as made obvious by larger atomic motions and stronger fluctuations in the length of the hydrogen bonds. Complete unfolding of the helices was not observed on a 200 ps timescale. The root mean square deviation (RMSD) values of the backbone atoms after 200 ps simulation compared to the starting structure underline the strong influence of the solvent on the protein stability. This RMSD value is 1.95 A for the simulation in water and 1.29 A for the simulation in TFE/water. This result supports the notion that TFE acts as a secondary structure inducing and stabilizing solvent. The differences apparent from the MD simulations are in good agreement with the data derived from NMR measurements, showing the relevance of MD as a method for estimating conformational and dynamical properties of proteins.

Homology modeling of adenylosuccinate synthetase from Saccharomyces cerevisiae reveals a possible binding region for single-stranded ARS sequences.

Adenylosuccinate synthetase from Saccharomyces cerevisiae was investigated in order to find a structural explanation for its ability to bind specifically to single-stranded ARS elements (autonomously replicating sequences). Using the E. coli enzyme as template, a model for the structure of adenylosuccinate synthetase from S. cerevisiae was generated and subsequently refined by molecular dynamics techniques. The resulting three-dimensional structure offers an explanation for the DNA binding activity of the yeast enzyme by revealing a distinct basic region that is not present in the homologous enzymes from other organisms. The model is also in good agreement with biochemical data available for a mutant protein in which Glycine 252 is replaced by Aspartate. On the basis of the model a significant structural distortion near the catalytic center was predicted for this mutant, corresponding well to the enzymatic inactivity observed. The mutant enzyme shows larger structural fluctuations than the wild-type protein according to the results of two independent molecular dynamics simulations.

Structural model of the HIV-1 Tat(46-58)-TAR complex.

The trans-activator protein (Tat) of human immunodeficiency virus type 1 (HIV-1) binds to an uridine-rich bulge of an RNA target (TAR; trans-activation responsive element) predominantly via its basic sequence domain. The structure of the Tat(46-58)-TAR complex has been determined by a novel modeling approach relying on structural information about one crucial arginine residue and crosslink data. The strategy described here solely uses this experimental data without additional "modeling" assumptions about the structure of the complex in order to avoid human bias. Model building was performed in a fashion similar to structure calculations from nuclear magnetic resonance (NMR)-spectroscopic data using restrained molecular dynamics. The resulting set of structures of Tat(46-58) in its complex with TAR reveals that all models have converged to a common fold, showing a backbone root mean square deviation (RMSD) of 1.36A. Analysis of the calculated structures suggests that HIV-I Tat forms a hairpin loop in its complex with TAR that shares striking similarity to the hairpin formed by the structure of the bovine immunodeficiency virus Tat protein after TAR binding as determined by NMR studies. The outlined approach is not limited to the Tat-TAR complex modeling, but is also applicable to all molecular complexes with sufficient biochemical and biophysical data available.

Functional null mutations in the gonosomal homologue gene TBL1Y are associated with non-syndromic coarctation of the aorta.

In patients with congenital heart defects, chromosomal anomalies are 100 times more frequent than in control subjects. Coarctation of the aorta can be detected in 15-20% of patients with Ullrich-Turner syndrome. By extensively reviewing literature involving breakpoint analysis of gonosomal deletions in Ullrich- Turner syndrome patients with and without coarctation of the aorta, we identified several gonosomal homolgous gene pairs of interest. Four of these homologous gene pairs were investigated by standard DNA sequencing in a cohort of 83 patients with non-syndromic coarctation of the aorta. Subsequently stability of mutant RNA and protein was analyzed to verify functional relevance of detected mutations. We identified two unreported missense mutations in Exon 8 (p.D69H) and 9 (p.R176W) of TBL1Y. Bioinformatic analysis and 3D modelling predicted that both mutations lead to TBL1Y loss of function. In RT-PCR and Western blot analyses of HEK293 cells transfected with a vector carrying the full-length TBL1Y (wild-type and mutant), we documented the predicted protein instability by showing protein decay for both mutant proteins. TBL1Y is similar to its gonosomal homologue, TBL1X, and its autosomal homologue, TBLR1, on chromosome 3. Both genes are part of co-repressor machineries and required for transcriptional activation by transcription factors that involve CtBP1/2, which contributes to Notch signaling. Several studies have shown that Notch signalling is important for proper development of the left ventricular outflow tract. Our findings suggest that TBL1Y is involved in the genesis of non-syndromic coarctation of the aorta.

Relevance of conserved lysine and arginine residues in transmembrane helices for the transport activity of organic anion transporting polypeptide 1B3.

BACKGROUND AND PURPOSE: Organic anion transporting polypeptide 1B3 (OATP1B3) (SLCO1B3) mediates the uptake of endogenous substrates (e.g. estrone-3-sulphate) and drugs (e.g. pravastatin) from blood into hepatocytes. Structure-based modelling of OATP1B3 suggested that a pore with a positive electrostatic potential contributes to the transport mechanism. Therefore, we investigated the role of conserved positively charged amino acids for OATP1B3-mediated uptake of sulphobromophthalein (BSP) and pravastatin. EXPERIMENTAL APPROACH: Residues Lys28, Lys41 and Arg580 in OATP1B3 were substituted by alanine, arginine, glutamine, glycine or lysine. Using immunofluorescence, immunoblot analysis and cellular uptake assays, the effect of these mutations on protein expression and transport activity was investigated. KEY RESULTS: Immunofluorescence revealed that all mutants were localized in the plasma membrane with partial intracellular retention of the Arg580>Ala and Arg580>Lys mutants. Lys41>Ala, Lys41>Gln, Lys41>Gly, Arg580>Gly and Arg580>Lys showed significantly reduced transport for BSP and pravastatin. Kinetic analyses of BSP transport revealed a significant reduction of V(max) normalized to cell surface protein expression for Lys41>Ala (wild type: 190 +/- 8, Lys41>Ala:16 +/- 4 pmol (mg protein)(-1) min(-1), P < 0.001), whereas V(max) of Lys41>Arg and Arg580>Lys (103 +/- 8 and 123 +/- 14 pmol (mg protein)(-1) min(-1), P > 0.05) did not change significantly. This suggests that the positive charges at positions 41 and 580 are important for transport activity of BSP. Structural modelling indicated that the positively charged side chain of Lys41 is flexible within the pore. The orientation of Arg580 is defined by adjacent residues Glu74 and Asn77, which was confirmed by kinetic analysis of Glu74>Ala. CONCLUSIONS AND IMPLICATIONS: We demonstrated that the conserved positively charged amino acids Lys41 and Arg580 are pivotal to the transport activity of OATP1B3.

Disturbed Wnt Signalling due to a Mutation in CCDC88C Causes an Autosomal Recessive Non-Syndromic Hydrocephalus with Medial Diverticulum.

The etiology of non-syndromic hydrocephalus is poorly understood. Via positional cloning in a consanguineous family with autosomal recessive hydrocephalus we have now identified a homozygous splice site mutation in the CCDC88C gene as a novel cause of a complex hydrocephalic brain malformation. The only living patient showed normal psychomotor development at the age of 3 years and 3 months and her deceased aunt, who was assumed to suffer from the same condition, had mild mental retardation. The mutation in the affected patients, a homozygous substitution in the donor splice site of intron 29, resulted in a shorter transcript due to exclusion of exon 29 and loss of functional protein, as shown by Western blotting (p.S1591HfsX7). In normal human tissue panels, we found CCDC88C ubiquitously expressed, but most prominently in the fetal brain, especially in pons and cerebellum, while expression in the adult brain appeared to be restricted to cortex and medulla oblongata. CCDC88C encodes DAPLE (HkRP2), a Hook-related protein with a binding domain for the central Wnt signalling pathway protein Dishevelled. Targeted quantitative RT-PCR and expression profiling of 84 genes from the Wnt signalling pathway in peripheral blood from the index patient and her healthy mother revealed increased mRNA levels of CCDC88C indicating transcriptional upregulation. Due to loss of CCDC88C function ß-catenin (CTNNB1) and the downstream target LEF1 showed increased mRNA levels in the patient, but many genes from the Wnt pathway and transcriptional target genes showed reduced expression, which might be explained by a complex negative feedback loop. We have thus identified a further essential component of the Wnt signalling pathway in human brain development.

Molecular Dynamics Simulation of Equine Infectious Anemia Virus Tat Protein in Water and in 40% Trifluoroethanol.

Abstract Two molecular dynamics (MD) simulations were performed in order to increase the understanding of the dependence of protein conformation on solvent environment. The protein used for these simulations is the transcriptional activator of the equine infectious anemia virus (EIAV-Tat). The structure of this protein has been determined by nuclear magnetic resonance (NMR) in aqueous solution (Willbold et al., Science 264, 1584 (1994)) and in 40% (v/v) trifluoroethanol (TFE) (Sticht et al., Eur. J. Biochem., submitted) showing considerable differences in the stability of the secondary structure elements. In order to investigate the influence of the solvent MD simulations (300 K; 200 ps) were carried out in water and in a solvent containing 40% (v/v) TFE. In both simulations the structure as determined in 40% TFE by NMR, showing three-helices and a tight type II turn, was used as the initial structure. The MD simulations clearly indicate a decreased stability of the secondary structure elements in aqueous environment as made obvious by larger atomic motions and stronger fluctuations in the length of the hydrogen bonds. Complete unfolding of the helices was not observed on a 200 ps timescale. The root mean square deviation (RMSD) values of the backbone atoms after 200 ps simulation compared to the starting structure underline the strong influence of the solvent on the protein stability. This RMSD value is 1.95 Å for the simulation in water and 1.29 Å for the simulation in TFE/water. This result supports the notion that TFE acts as a secondary structure inducing and stabilizing solvent. The differences apparent from the MD simulations are in good agreement with the data derived from NMR measurements, showing the relevance of MD as a method for estimating conformational and dynamical properties of proteins.